The present invention relates to a laser beam frequency stabilization method and apparatus and more particularly to a system for dispersing a laser beam to generate an error signal which is used to stabilize the frequency of the laser beam.
A light beam may be amplified by stimulated emission of radiation to produce what is commonly referred to as a laser (light amplification by stimulated emission of radiation) beam. A laser beam is produced in an active gain medium which is constrained by a resonant cavity structure.
The lasing medium within the cavity is stimulated to produce light at frequencies determined by certain electronic and thermodynamic characteristics of the lasing material. The active medium is typically an excited gas, such as carbon dioxide, hydrogen fluoride, carbon monoxide, neon, helium or rhodamine dye. As the light travels through the active medium, its intensity increases to thereby provide amplified radiation. The means for exciting the medium are typically a flash lamp for ruby or yag glass, electrical discharge in a gas, electrical currents, in the case of a semi-conductive device, or a chemical reaction in the case of hydrogen fluoride.
The output frequency of the laser beam is determined by the optical length of the laser cavity. A cavity having a longitudinal length of n1/2 (where n is an integer and 1 is the wavelength) produces a laser beam having a wave length equal to 1.
A laser cavity typically includes a pair of mirrors each at opposite ends of the active medium to form the resonant cavity having a length equal to n1/2. The radiation is reflected back and forth between the mirrors. The mirror radii, the ratio of the effective apertures of the cavity to its length and the transmission of the mirrors determine the losses which must be compensated for by the gain provided by the active medium to cause the light amplification to reach a sufficiently high level to emit radiation. When the active medium compensates sufficiently for the various losses in the structures so that the light amplification begins to emit radiation, the structure is said to be lasing. The length of the resonant cavity structure formed by the mirrors at opposite ends is therefore determinative of the wavelength of the light beam which is amplified to emit radiation.
Laser beams are frequently used for isotope separation, radar and heterodyne measurements, spectroscopic work, and communication. In order for laser beams to be used in such applications, lasers must have a stable, controllable wave length and frequency and a stable amplitude. The operation must be with a stable laser.
In normal laboratory laser oscillators, the cavity length is not constant but is perturbed by acoustic vibrations, thermal changes and other external influences. These changes occur basically at random. A number of approaches to laser frequency stabilization have been previously employed, including frequency lock to a fixed absorption line in an external gas cell, and dither stabilization to a center of a Lamb dip in the laser power tuning characteristic.
Frequency stabilized lasers using an external gas cell which provides an absorption resonance is shown in U.S. Pat. No. 3,921,099 to Abram et al. This technique, however, can only be used at frequencies where there is a suitable Stark effect and is limited in its control of tunability to a narrow frequency range.
Laser frequency control by use of the Lamb dip effect suffers a serious drawback. If a single frequency signal is swept about its center, there will be an amplitude minimum at the center of the frequency. This minimum is called the Lamb dip and the small rises in amplitude on either side of the dip are usable as directional control error signals. Such a technique can be used to lock a laser cavity to a single frequency. However, this system can be made to lose control of a preferred frequency. In the case of a gas laser, a strong electrical transient may suddenly shift the operating frequency to another frequency to which it will then be locked. Thus, the Lamb dip technique may be said to be frequency blind.
Laser stabilizing systems using multifrequency techniques are shown in U.S. Pat. Nos. 3,500,236 to George L. Clark and 3,487,327 to Peter O. Clark.
In such systems, two stable frequencies are established in a single laser cavity and an error signal is obtained in response to a variation in the change in frequencies. Such a system is difficult to implement since the cavity length is subject to severe geometric constraints. Also, since such systems must support laser beams having several frequencies, they are inefficient.
A laser frequency stabilizing device has been developed that employs variations in the position of interference patterns which result from variations in the output frequency of a laser. These interference patterns are generated by sampling a portion of a laser output and passing it through a Fabry Perot etalon, which spatially separates the light as a function of frequency. The error signal generated by detecting the spatial changes in these patterns is used to control the optical length of the cavity as shown in U.S. Pat. No. 3,967,211 to Itzkan et al. Such a device is designed for use in a dye laser for the separation of isotopes and requires an elaborate temperature control equipment associated with the etalon.
Furthermore, all of the devices described above require an excessive amount of circuitry and hardware which renders them costly and prone to failure.